On Tue, Mar 21, 2023 at 4:28 PM Kui-Feng Lee <kuifeng@xxxxxxxx> wrote:
>
> Introduce bpf_link__update_map(), which allows to atomically update
> underlying struct_ops implementation for given struct_ops BPF link
>
> Signed-off-by: Kui-Feng Lee <kuifeng@xxxxxxxx>
> ---
> tools/lib/bpf/bpf.h | 5 ++++-
> tools/lib/bpf/libbpf.c | 35 +++++++++++++++++++++++++++++++++++
> tools/lib/bpf/libbpf.h | 1 +
> tools/lib/bpf/libbpf.map | 1 +
> 4 files changed, 41 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> index f0f786373238..4fae4e698a8e 100644
> --- a/tools/lib/bpf/bpf.h
> +++ b/tools/lib/bpf/bpf.h
> @@ -335,7 +335,10 @@ LIBBPF_API int bpf_link_detach(int link_fd);
> struct bpf_link_update_opts {
> size_t sz; /* size of this struct for forward/backward compatibility */
> __u32 flags; /* extra flags */
> - __u32 old_prog_fd; /* expected old program FD */
> + union {
> + __u32 old_prog_fd; /* expected old program FD */
> + __u32 old_map_fd; /* expected old map FD */
> + };
so for these low-level wrappers in libbpf with OPTS we've been trying
to avoid unnecessary unions. If you look at bpf_link_create and
bpf_link_create_ops some fields that are in a union in kernel UAPI are
actually listed as separate fields, and libbpf makes sure that both
fields are not specified at the same time (like iter_info_len and
target_btf_id, for instance).
So let's do the same here, instead of making a union, let's have
__u32 old_prog_fd;
__u32 old_map_fd;
and then in bpf_link_update() implementation make sure that both can't
be set at the same time.
The rest of the patch looks good to me, thanks.
> };
> #define bpf_link_update_opts__last_field old_prog_fd
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 3b257d5170cb..935a7da501d7 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -11670,6 +11670,41 @@ struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map)
> return &link->link;
> }
>
> +/*
> + * Swap the back struct_ops of a link with a new struct_ops map.
> + */
> +int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map)
> +{
> + struct bpf_link_struct_ops *st_ops_link;
> + __u32 zero = 0;
> + int err;
> +
> + if (!bpf_map__is_struct_ops(map) || map->fd < 0)
> + return -EINVAL;
> +
> + st_ops_link = container_of(link, struct bpf_link_struct_ops, link);
> + /* Ensure the type of a link is correct */
> + if (st_ops_link->map_fd < 0)
> + return -EINVAL;
> +
> + err = bpf_map_update_elem(map->fd, &zero, map->st_ops->kern_vdata, 0);
> + /* It can be EBUSY if the map has been used to create or
> + * update a link before. We don't allow updating the value of
> + * a struct_ops once it is set. That ensures that the value
> + * never changed. So, it is safe to skip EBUSY.
> + */
> + if (err && err != -EBUSY)
> + return err;
> +
> + err = bpf_link_update(link->fd, map->fd, NULL);
> + if (err < 0)
> + return err;
> +
> + st_ops_link->map_fd = map->fd;
> +
> + return 0;
> +}
> +
> typedef enum bpf_perf_event_ret (*bpf_perf_event_print_t)(struct perf_event_header *hdr,
> void *private_data);
>
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index db4992a036f8..1615e55e2e79 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -719,6 +719,7 @@ bpf_program__attach_freplace(const struct bpf_program *prog,
> struct bpf_map;
>
> LIBBPF_API struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map);
> +LIBBPF_API int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map);
>
> struct bpf_iter_attach_opts {
> size_t sz; /* size of this struct for forward/backward compatibility */
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 50dde1f6521e..a5aa3a383d69 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -386,6 +386,7 @@ LIBBPF_1.1.0 {
> LIBBPF_1.2.0 {
> global:
> bpf_btf_get_info_by_fd;
> + bpf_link__update_map;
> bpf_link_get_info_by_fd;
> bpf_map_get_info_by_fd;
> bpf_prog_get_info_by_fd;
> --
> 2.34.1
>
